Influence of string-box size and the derating factor associated to the DC protection for PV solar panels.

https://doi.org/10.1016/j.epsr.2022.107992Get rights and content

Highlights

  • A new method and calculation for the DC protection design for solar plants.

  • It studies the influence of the derating factors, loading cycle, string-box caused by irradiance variation for clouds.

  • A case study with laboratory test and new algorithms have an accuracy of 99.99%.

  • It improves the heat transfer in the string-box with 111% with KZS is 74.14% to KZS = 82.3%.

Abstract

In the last decade, the photovoltaic (PV) solar plants have been spreading around the world with low cost and a construction record time. In this perspective, the challenges have been centered in the inverters, panels, and medium voltage cables design, in order to obtain the best efficiency; however, one of the main elements is the fuse for the strings, in the DC side. Several manufacturers have detected problems in the operating regime of fuses due to the recommendations of the IEC standard 61439-1, 61439-2, 60269-6, 60269-1, 60469; our findings are the influence of the derating factors, loading cycle of the fuse in PV solar plants caused by clouds and string-box size; it has influence in the fuse design; a traditional IEC calculation is not an adequate value for the DC fuse, with fuses melt until 30% of the total fleet per year, in a regular PV Solar plant, but with a new consideration of string-box size and derating factor in the fuse design; with this methodology, the failure rate could be reduced, suddenly, with a result of to 0.2% failures per year, with an improvement of 1.06% of the derating factor and size of the string-box, with the algorithm for future development and design according the best performance in the PV solar plant as a complementary procedure in the international standard with an accuracy of 99.99%.

Introduction

The fuses have the main function of protecting a component or element of the electrical system against the damages that the overcurrent can produce, so the magnitude and duration of the overcurrent protection, it circulates through the protected device that must be controlled by the fuse, so that the protected item is not damaged [1].

The high capacity fuse for direct current presents particularities in its behavior, critical in the application of solar power plants [2], battery energy storage systems banks and capacitors bank, which should be understood if a correct operation of the protection scheme is intended, four particularities are identified which are the following: following: (i) Difficulties in interrupting low overcurrent; (ii) difficulty in interrupting direct current; (iii) difficulty in current interruption with low inductance/resistance ratio and; (iv) premature aging due to cyclical overloads [3,4].

In this way, the fuse was born as a device to act against high overcurrent, of the short-circuit type [5,6]. The extension of its operation to slight overloads occurs with the so-called "M effect", which consists of a deposit of material with a low melting point on the fuse element, usually copper or silver [7]. This added material lowers the starting temperature of the fusion process from 1083 °C or 960 °C of copper or silver to 300 °C, thus achieving the start of the operation with overcurrent of the order of 125% of the fuse rating [8]. If the current is less than that percentage, then the fuse won´t actuate. This clearly shows the difficulty in this application where fault currents are extremely low [9].

On the other hand, cyclical overloads have a real problem in traditional fuses, about the M effect, it represents a cumulative process. In other words, the presence of the overburden initiates the dissolution process of the base sheet by means of the low melting point alloy [10]. If the overload disappears, the dissolution phenomenon ceases, leaving the fuse element with a smaller conductive section. As it has a smaller section, its nominal current is now lower, so the nominal load current is taken as an overload that advances the dissolution process. This behavior leads to cyclical overloads aging the fuse prematurely, leading to untimely operation [11].

In the application of solar power plants, gPV fuses are used, as the demand for photovoltaic cells and the number of installations increases, the need for their effective protection against electrical transients, such as short circuits, overloads, reverse currents and overvoltage, also grows rapidly. These new requirements have led to the design, development and marketing of a new series of special fuses for direct current protection, whose class is gPV [12]. The international standard governing design is IEC 60269-6 [3]. The main characteristics are the following. The nominal current is evaluated according the international standard IEC 60269-1 [4]; in this case, the cycle loading should be higher than 3000 cycles, without the fuse characteristics. The fuses’ average load is not usually higher than 70% or 80% of the nominal current, therefore, derating (additional derating) is not required when six or more circuits are close together, in case of using high loss fuses, according the circuits:

  • Two to four circuits with a derating factor of 0.9 [5].

  • Five to six circuits with a derating factor of 0.8 [5].

  • Six to nine circuits with a derating factor of 0.7, and higher than ten circuits have a derating factor of 0.6 [5].

  • The nominal current of the fuse is determined for 25 °C, the cells are normalized for the same value, but can operate at higher temperatures, therefore the fuse must be derated (declassified), by means of correction factors.

Finally, the requirements for suitable fuses for the protection of photovoltaic cells are the following:

  • Normal fuse voltage at least equal to 1.2 Uoc, applicable up to 1500 Vdc, which allows working under extreme conditions such as temperatures down to -25 °C [13].

  • Fuse rated current up to 25 A, for chain protection fuse and up to 400 A for capacitor bank protection fuses.

  • Safe interruption of low fault currents.

  • It is mandatory to have full-range protection characteristics, gPV class.

  • Fast operation.

  • Resistant against cyclical load.

  • Low power losses.

  • Compact dimensions.

Today, the new large PV solar plants are increasing in size and optimization with the biggest conversion units, inverter, string-box, cables, and panels; however, the string-box and fuses melt have been increased, and its impact in the root cause analysis. It is considered the sixth failure mode out of eighteen in 2020 [15].

Even in a smart grid application, two boost or buck inverters could be changed by a CUK-SEPIC and CUK-Buck fused, with less efficiency in an overcurrent in a frequency regulation stage, but it allows more capacity in a cheapest cost, it compared with the traditional AC/DC inverter for hybrid solutions; the problem is the highest current in the DC side and it doesn´t consider aspect as thermal dissipation, string-box size, fusion time of the fuse associated to the irradiance (normal or extreme solar overirradiance), derating coefficient; it should be additional to the traditional constraints as short circuit and nominal current, curve characteristic, load cycle, ambient temperature [16]. An especial attention is the irradiance and the influence over the fuses, due to long overirradiance over five minutes as a period, the results are a “very high fuse operating temperatures pre-vailing in the field, deleterious consequences were evaluated with the blowing of even slow-blow fuses” [17]; therefore, a new proposal for the fuses and string-box design is required, in order to avoid this malfunction and massive blowing events.

This research article has the following sections: In the Section 2, the description of the failure and impacts, traditional calculation, and proposal. In the Section 3, the case study is described with the laboratory test, later, in the Section 4, the discussion about the new improvements and consideration of the international standards. Finally, the conclusions and the recommendation in the Section 5.

Section snippets

Description of the failure and impacts

A high failure rate of the string fuses has been registered from the beginning of operation of many PV solar plants; according to the evaluation of technical reports and thermal images about the thermal test and verify the proper sizing of the fuses [14]; in this case, the string-box and accessories produced by the manufacturers; in some cases, the international standards are IEC 61439-1 [1] and CEI-EN 61439-2 [2]; inside the climatic chamber available; in this case, the electrical parameters

Case study

In order to understand the conditions of the environment, the case study was developed in Peru, with the following meteorological station during 2021 with the Table 2.

Discussion

This research article has direct contribution and application in real life, specifically in solar projects based on photovoltaic cells. The results of the simulations carried out during the development of the research show a very critical situation in relation to the useful life, performance, and efficiency of photovoltaic projects. It presents an unprecedented practical idea that improves these aspects in a very refined way, which is directly focused on a fundamental component of all

Conclusions

The high capacity of the fuse for direct current is critical in the PV solar plants caused by interrupting low overcurrent, direct current, low inductance and premature aging due to cyclical overloads. The cyclical overloads affect in a sudden fuse melt with an early aging with the contribution of electrical transients, such as short circuits, overloads, reverse currents and overvoltage; therefore, the international standard IEC 60269-6 [3] should be improved with the following seven steps:

  • To

Grants/financial support

None.

CRediT authorship contribution statement

R.M. Arias Velásquez: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing – original draft, Writing – review & editing, Visualization, Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Acknowledgment

Recognition to Universidad Tecnológica del Perú.

References (20)

  • G.L. Martins et al.

    Evaluating the performance of radiometers for solar overirradiance events

    Sol. Energy

    (2022)
  • H. Zsiborács

    Assessing shading losses of photovoltaic power plants based on string data

    Energy Rep.

    (2021)
  • IEC, IEC 61439-1, “Low-voltage switchgear and controlgear assemblies-part 1: general rules”, Edition 3.0, 2020 05,...
  • IEC, IEC 61439-2, “Low-voltage switchgear and controlgear assemblies-part 2: power switchgear and controlgear”, Edition...
  • IEC, IEC 60269-6 Low-voltage fuses - part 6: supplementary requirements for fuse-links for the protection of solar...
  • IEC 60269-1:1968, Low-voltage fuses with high breaking capacity for industrial and similar purposes - part 1: general...
  • IEC 60469:2013, Transitions, pulses and related waveforms-terms, definitions and algorithms, 2013,...
  • UL 2579, Outline of investigation for low-voltage fuse-fuses for photovoltaic systems, 2013,...
  • V. Vanitha et al.

    Hybrid wind and solar based battery charging controller

  • L.D. Murillo-Soto et al.

    Fault detection in solar arrays based on an efficiency threshold

There are more references available in the full text version of this article.

Cited by (3)

  • Converting data into knowledge with RCA methodology improved for inverters fault analysis

    2022, Heliyon
    Citation Excerpt :

    Furthermore, the qualitative design has considered the 31 factors for the evaluation before (2019) and after (2021). The root cause analysis has detected a bad signal programmed in the DC/AC converter, associated to the PLL devices, it caused an overcurrent in the inverter, and a sudden disconnection, sometimes with massive fuses melt during high irradiance periods [28]. The overload of the dc/ac converter caused by the overcurrent; it has a sudden frequency variation of 0.3 Hz; therefore, it was validated with the detection of the root cause, and the evaluation of the qualitative research with the improvement of the 140%.

  • Methodology for Metal Oxide Varistor design applied to Fixed series capacitor

    2022, Proceedings of the 2022 IEEE Engineering International Research Conference, EIRCON 2022
  • Systematic review of the automation of a stacking system for wood drying

    2022, Proceedings of the 2022 IEEE 29th International Conference on Electronics, Electrical Engineering and Computing, INTERCON 2022
View full text